3. The time, T seconds, it takes a water heater to boil some water is directly proportional
to the mass of water, mkg, in the water heater.
When m= 250, T = 600
(a)
Find T when m= 400
T=
(3)

T = 960 seconds

Step-by-step explanation:

the equation of the direct proportion is y = k x, where k is the constant of variation, we find it using the initial amount of x and y

∵ T is the time in seconds

∵ m is the mass of water in kg

∵ The time, T it takes a water heater to boil some water is directly

   proportional to the mass of the water, m

∴ T = km

∵ When m = 250 kg, T = 600 seconds

→ Substitute them in the equation above to find k

∵ 600 = k(250)

→ Divide both sides by 250 to find k

∴ 2.4 = k

→ Substitute the value of k in the equation above

∴ T = 2.4m ⇒ the equation of the direct proportion

∵ m = 400 kg

→ Substitute it in the equation of variation to find T

∵ T = 2.4(400)

∴ T = 960 seconds